What is cholesterol?

Cholesterol is a fatty substance (a lipid) that is an important part of the outer lining (membrane) of cells in the body of animals. Cholesterol is also found in the blood circulation of humans. The cholesterol in a person’s blood originates from two major sources, dietary intake and liver production. Dietary cholesterol comes mainly from meat, poultry, fish, and dairy products. Organ meats, such as liver, are especially high in cholesterol content, while foods of plant origin contain no cholesterol. After a meal, cholesterol is absorbed by the intestines into the blood circulation and is then packaged inside a protein coat. This cholesterol-protein coat complex is called a chylomicron.

The liver is capable of removing cholesterol from the blood circulation as well as manufacturing cholesterol and secreting cholesterol into the blood circulation. After a meal, the liver removes chylomicrons from the blood circulation. In between meals, the liver manufactures and secretes cholesterol back into the blood circulation.

What is LDL and HDL cholesterol?

Cholesterol, like oil, cannot dissolve in the blood unless it is combined with special proteins called lipoproteins. (Without combining with lipoproteins, cholesterol in the blood will turn into a solid substance.) The cholesterol that is secreted by the liver into the blood is combined either with very low-density lipoproteins (VLDL) or high-density lipoproteins (HDL). VLDL cholesterol is then metabolized in the bloodstream to produce LDL cholesterol. The cholesterol combined with low-density lipoprotein is called LDL cholesterol, and the cholesterol combined with high-density lipoproteins is called HDL cholesterol.

How does cholesterol cause heart disease?

Heart Attack illustration - Cholesterol and The Heart

LDL cholesterol is called “bad” cholesterol, because elevated LDL cholesterol is associated with an increased risk of coronary heart disease. LDL lipoprotein deposits cholesterol on the artery walls, causing the formation of a hard, thick substance called cholesterol plaque. Over time, cholesterol plaque causes thickening of the artery walls and narrowing of the arteries, a process called atherosclerosis. Arteries that supply blood and oxygen to the heart muscles are called coronary arteries. When coronary arteries are narrowed by atherosclerosis, they are incapable of supplying enough blood and oxygen to the heart muscle during exertion. Lack of oxygen (ischemia) to the heart muscle causes chest pain, also formation of a blood clot in the artery can clause complete blockage of the artery, leading to death of heart muscle (heart attack). Atherosclerotic disease of coronary arteries (coronary heart disease) is the most common cause of death in the United States, accounting for about 600,000 deaths annually.

HDL is called the “good cholesterol” because HDL cholesterol particles prevent atherosclerosis by extracting cholesterol from the artery walls and disposing of them through the liver. Thus, high levels of LDL cholesterol and low levels of HDL cholesterol (high LDL/HDL ratios) are risk factors for atherosclerosis, while low levels of LDL cholesterol and high level of HDL cholesterol (low LDL/HDL ratios) are desirable.

What determines the level of LDL cholesterol in the blood?

The liver not only manufactures and secretes LDL cholesterol into the blood, it also removes LDL cholesterol from the blood. To remove LDL cholesterol from the blood, the liver relies on special proteins called LDL receptors that are normally present on the surface of liver cells. LDL receptors snatch LDL cholesterol particles from the blood and transport them inside the liver. A high number of active LDL receptors on the liver surfaces is associated with the rapid removal of LDL cholesterol from the blood and low blood LDL cholesterol levels. A deficiency of LDL receptors is associated with high LDL cholesterol blood levels.